A stone on a table won't budge without a kick. Where does this stubbornness come from? The idea that the rest of the Universe is to blame was put forward over a century ago. Now it has been formalized into a rigorous theory.
In the new work, gravity is described not as an attraction between bodies but as a field that sets the scale of distances and time. All matter in the Universe creates this field. A body's inertia is its response to the field created by all matter in the past. The more matter around, the stronger the body resists acceleration.
For ordinary stones and planets, the theory gives the same results as conventional physics. But for very dense objects, like neutron stars, differences appear. These can be tested with observations.
Thus, inertia ceases to be a mysterious property of matter. It becomes the Universe's memory of itself.
as written by the authors arXiv:2601.07904
We formulate a scalar realization of Sciama's Machian programme within the general Bergmann-Wagoner class of scalar--tensor gravity. Starting from a universally conformally coupled matter sector, we rewrite the field equations in terms of the invariant set $$\{{\cal I}_1,{\cal I}_2,{\cal I}_3,\hat g_{\mu u}\}$$, so that Machian requirements can be stated independently of conformal frame. Sciama's causal postulate is implemented not by modifying the local dynamics, but as a selection rule on the solution space of the invariant scalar equation: the admissible configuration is the retarded response to the matter distribution in the causal past, with any source-free contribution removed. In a spatially flat FLRW universe, and in the light, slowly varying regime, the prescription reduces to an explicit temporal kernel that links the background scalar evolution to the matter content within the Hubble region, reproducing the expected Machian scaling in an expanding background. Universal coupling to a single physical metric implies that structureless test bodies share the same acceleration in a given external configuration, so that the E\"otvs parameter vanishes at leading order. Nonuniversal effects can arise only when gravitational binding energy contributes appreciably to the total mass, as in strongly self-gravitating objects. Thus, the theory implements a causal Machian determination of the cosmological inertial scale while remaining consistent with standard weak-field tests.
If the Universe were empty, bodies would have no inertia: they would accelerate from the slightest push without resistance.